The invention relates to a filter medium for solid/liquid separation, preferably as a deposit support for a deposit filter and for cake-forming filtration according to the type specified in the preamble of claim 1.
The operating principle of deposit filters includes the initial deposition of a filter layer which extends across the openings of a deposit support. First, a base deposition is carried out to form a first pre-deposit by flowing the deposit material in a fast circulation through the filter so that in this manner a thin layer of particles is formed on the filter surfaces. Since the deposit material, in many applications silica gel, has a particle spectrum of 0.3 .mu.m to approximately 50 .mu.m, and the deposit support has usually openings of a magnitude of 60 .mu.m to 100 .mu.m, the deposit layer can be formed only when a plurality of particles become wedged in the opening so as to form a bridge so that thereby penetration of the opening is hindered. With the second initial deposition a cake of a layer thickness of 0.5 mm to 1.0 mm is then built.
According to the operating principle of a cake-forming filtration, foreign filter aids that are deposited onto a filter medium are completely unnecessary. The solids which are contained in the suspension to be filtered grow directly on the surface of the filter medium to a deposit or filter cake while the filtrate is removed through the filter medium.
Depending on the field of application of the filter different filter media can be used. Deposit supports comprised of a plastic fabric or a metal fabric as well as slotted sieves are the most common embodiments for a deposit filtration. For filters such as single layer filters or suction filters or single layer process filters or dry filters it is also known to employ sinter materials such as, for example, multi-layer sinter fabrics, sinter fleece, or sintered powder sheets. An opening magnitude of approximately 55 .mu.m is the lower limit of practical, useful structures in the known fabric embodiments because for a mesh width that is smaller the fibers would have to be so thin that the rip stability no longer would be sufficient. Moreover, the ratio of the surface area of the openings to the total surface area is fixed and not variable.
It is an object of the present invention to provide a filter medium for the solid/liquid separation that, even for low particle size or fine particle size distribution of the filter aid, quickly and reliably forms a stable uniform filter layer across the entire filter surface.